The present invention relates to a relatively inexpensive and simplified magnetic immunity system which fully compensates for the effects of primarily the axial component of an ambient magnetic field to which the system is exposed, and which effects automatic quick-degaussing as necessary. The invention also relates to a monitor, in particular a color monitor, which includes such a magnetic immunity system, and to a method of compensating for the effects of an ambient magnetic field on the cathode ray tube (CRT) of such a color monitor.
Rather extensive efforts have now been made with the goal of neutralizing the effects that a substantial ambient magnetic field has on a cathode ray tube (CRT) color monitor. There are three primary effects of such a magnetic field on a CRT color monitor. These effects are now well recognized. First, the field may cause the electron beam to focus on an incorrect phosphor area. In a color monitor, the electron beam which is, for example, determined to target a red phosphor, could be driven by a strong ambient field to impinge on a blue or green phosphor area. This results in a change in the image color, and thus a color purity error. This may be the worst result caused by exposure to such ambient fields. A second problem is when the field causes the entire image to shift on the screen (image translation). When image translation is caused by a static field, compensation is rather simple. When the shift is caused by alternating magnetic fields, without compensation, the screen image will appear to wobble. The third general category of errors caused by ambient fields, particular very strong fields, is convergence error. This is where a white line appears as split into three colored lines, i.e. red, green and blue.
There have been several approaches to compensation for the effects of an ambient magnetic field on CRT monitors. Modern approaches involve a passive magnetic shielding for protection of areas of the CRT other than necessary for viewing. In addition to the shielding, compensation coils have been positioned about the monitor in order to produce offsetting magnetic fields. Teaching a manner of controlling such compensation coils, U.S. Pat. No. 4,380,716 has been understood as disclosing the generation of patterns at the corners of the monitor screen by the electron beam gun assembly. Then, changes in the pattern due to the changes in the ambient field are sensed by optical sensors to control current flow through compensation coils to provide for axial correction. A different approach is set out in U.S. Pat. No. 3,757,154 and involves magnetic sensors in a bridge configuration to control current flow through compensation coils.
Other systems have been disclosed in U.S. Pat. Nos. 4,963,789; 5,039,911; and 5,073,744. As understood from these patents, four magnetic sensors are provided in a plane to have parallel primary axes of sensitivity. Each of the sensors is associated with one of four corresponding compensation coils, and each coil is driven by a current applied thereto in accordance with an ambient magnetic field as sensed by its respective sensor. The coils have a common center. With this disclosed arrangement of four sensors and four corresponding coils, compensation is performed for intrusive vertical and lateral magnetic field components as well as the axial component. However, such a system is complicated. It is relatively expensive and requires a complicated calibration routine which would tend to lengthen repair time when necessary for such a machine.
Also in the context of prior efforts, U.S. Pat. No. 4,458,178 is recognized as disclosing a degaussing arrangement for a CRT. U.S. Pat. Nos. 3,340,443 and 3,340,307 are recognized as relating to a degaussing system, and a demagnetization system for a color television respectively.